1. Field of the Invention
The present invention relates to an inkjet recording apparatus for recording an image on a recording medium by ejecting ink droplets. Particularly, the present invention relates to an inkjet recording apparatus which ejects ink droplets by generating ultrasonic waves from an ultrasonic wave generation element and focusing the ultrasonic waves.
2. Description of the Related Art
An inkjet recording apparatus which records an image on a recording medium by ejecting liquid ink in the form of minute droplets has many advantages; for example, the capability of recording an image directly on plain paper, the capability of curtailing cost in terms of consumption of materials such as ink; low noise; and obviation of a necessity for processing such as development, fixing, and the like. Therefore, the inkjet recording apparatus has recently spread into wider application areas; for example, industrial fields such as application of liquid electronic materials, direct patterning, as well as in an office automation field.
Many mechanisms have already been contrived as the inkjet recording apparatus. Particularly typical mechanisms include a mechanism for ejecting droplets by utilization of pressure of bubbles generated by heat of a heating element, a mechanism for ejecting droplets by pressure pulses stemming from displacement of a piezoelectric element, and the like.
However, the pieces of inkjet recording apparatus having these mechanisms suffer a problem of condensation of ink being readily caused by evaporation or volatilization of a solvent of liquid ink and a problem of use of a nozzle of small bore resulting in occurrence of clogging which hinders ejecting of ink droplets. For these problems, image recording of particularly high definition requires provision of an additional unit for cleansing a nozzle to prevent occurrence of clogging of the nozzle. The range of usage of such an inkjet recording apparatus is confined because of a necessity for selectively using an ink material which causes less clogging.
In contrast, there has been proposed an inkjet recording apparatus of an acoustic mechanism for focusing ultrasonic waves stemming from a transducer and ejecting ink droplets from an ink reservoir level by sound pressure of the ultrasonic waves. This mechanism has an advantage of obviation of a necessity for a nozzle, the capability of ejecting ink droplets of very small diameter, the adaptability of the recording apparatus to higher resolution, and few restrictions on available ink materials.
However, highly-viscous ink such as that used for industrial applications generally involves great attenuation of ultrasonic waves and requires great power to eject ink. Further, in the case of ink of excessively-high viscosity, there arises a problem of a failure to eject ink. Fluctuations in the level of ink induce displacement in the position (an ultrasonic wave focus position) where the ultrasonic waves originating from the transducer are to be focused, thereby resultantly raising a problem of a failure to eject ink. In such a case, a mechanism for adjusting a head position to address the fluctuations in the level is required, which in turn results in complication of the recording apparatus. For avoiding such a problem, a technique for rendering an ink layer thin by use of a substance which propagates ultrasonic waves to an ultrasonic wave propagation path has been put forward, as disclosed in JP-A-6-238884.
According to the technique described in connection with JP-A-6-238884, ink must be fed to a thin layer. Hence, difficulty is encountered in feeding highly-viscous ink to a narrow clearance. Further, since there are many orifices, difficulty is also encountered in causing ink to flow over the head. In addition, the head can be oriented upward, but countermeasures must be taken against leakage of ink when the head is oriented downward, because of the plurality of orifices. Structurally, extreme difficulty is encountered in regulating the flow rate of ink when the head is oriented horizontally, because of a difference in pressure between an upper portion of the ink and a lower portion of the same.